This invention relates to a surface acoustic wave (SAW) device, and more particularly to a SAW resonator suitable for application to a broad-band oscillation circuit and the like. The SAW resonator according to the present invention may be used in a voltage-controlled oscillator having a broad variable frequency range or a cellular radio system, for example, a mobile telephone system or a cordless telephone system.
Prior art SAW resonators are discussed in IEEE Ultrasonics Symposium Proceedings, 1974, pp. 263-267. A one-port resonator described in the above publication has such a structure that a reflector formed by depositing an array of thin-film metal stripes on the surface of a substrate or engraving an array of grooves in the surface of the substrate is disposed on either side of a transducer provided on the substrate to produce surface acoustic waves.
In the prior art one-port resonator, the metal stripe array reflector has a narrow bandwidth, and the relative bandwidth, which is expressed as (f.sub.a -f.sub.r)/fr where f.sub.r and f.sub.a are the resonant frequency and the anti-resonant frequency respectively of the resonator, is generally in the order of only 0.01 to 1%. In the prior art one-port resonator, no consideration was given to the fact that the bandwidth of the resonator using such a reflector is further narrowed to about 1/2 to 1/6 of that of the reflector. Because of the lack of the above consideration, a resonator having a very narrow bandwidth could only be realized. Therefore, it has been difficult to realize a resonator which can be incorporated in a voltage-controlled oscillator in a cellular radio system such as a mobile telephone system which requires a bandwidth of about 20 to 40 MHz (a relative bandwidth of about 3 to 5%) in a frequency band of 800 MHz.